An automatic lance change device of the above mentioned kind is described in U.S. patent application, Ser. No. 07/843,129, inventor Hubert Stomp, assigned to Paul Wurth S. A., a Corporation of Luxembourg, hereinafter the '129 document which is specifically incorporated by reference herein.
In the device according to the '129 document, the coupling to the above mentioned lance carriage is effected with the aid of two lifting spindle systems, an external screw thread of a draw rod cooperating in each case with an internal screw thread in the bore of a gear wheel.
These gear wheels may be horizontally disposed wheels in the form of worm wheels, chain wheels, bevel gears, spiral-toothed gears or cogged-belt gears and others, which are mounted axially/radially by their hubs in a gear casing.
In the '129 document, worm wheels were shown by way of illustration in the pertaining figures and worms disposed on a common shaft were associated with them. In this connection the remark was added that these worms may however also be mounted on individual shafts which are connected together by means of a coupling for synchronous driving.
Although the overall concept of the device described in the '129 document constituted a substantial advance in relation to the prior art at the time, a disadvantage exist in that the two worm/worm wheel pairs (or pairs of any other gear wheels, as described hereinabove) are housed in a common casing, so that a gear unit of this type must obviously be specially produced. This common casing also entails the risk that displacements, which in practical operation are entirely possible, will result in the tilting of the two lifting spindle systems, with their possible destruction. Even the optional provision of a coupling, which is also mentioned, changes nothing in this respect as long as the two worm and worm wheel pairs are mounted and housed in a rigid common casing. An important reason for the provision of a coupling, however, is that a coupling enables the angle of rotation of the two worm shafts to be mutually adjusted on assembly, in order to ensure that the two lifting spindle systems will work with optimum synchronism, so that when the drive motor is automatically switched off these systems will press the lance against the coupling head with as precisely equal a force as possible on both sides.